The invention relates to a pulse radar apparatus comprising a transmitter and receiver, whereby the target returns--after detection--are sampled and digitised per range quant, an n-point FFT processing unit, and a threshold circuit for determining a threshold value for each of the n frequency output channels of the FFT processing unit, above which threshold value the output signals of the FFT processing unit are passed.
The FFT processing unit converts the video data, sampled and digitised per range quant, from n successive radar scans into n output signals situated in adjoining frequency bands. Such a conversion is hereinafter referred to as an FFT scan. A division of the radar range into radar cells formed by range quants and azimuth sectors determined by n successive azimuth scans then corresponds with a division into FFT cells formed by FFT scans and range quants. For the successive FFT scans the corresponding azimuth sectors may overlap each other partly; hence, in such a case the radar cells of these azimuth sectors will also overlap each other. The output signals of the FFT processing unit determine the spectrum of the target returns processed per radar cell.
With the use of a magnetron in the transmitter of the pulse radar apparatus the first trip echoes can be detected coherently, unlike the multiple trip echoes. The spectrum of the first trip echoes will therefore differ from that of the multiple trip echoes. With the radar beam moving over the target, the spectra of the coherent target returns, falling within successive radar cells in a range bin, differ mutually in the sense that the spectrum range is a minimum at the instant the beam is directed at the target centre and increases as the beam is directed away from the centre. In case the target has not yet reached the centre of the beam or has already passed that centre, the spectrum range of a coherent first trip echo may sometimes be greater than the spectrum range of multiple trip echoes, so that the coherent first trip echo may be classified as a multiple trip echo and be suppressed. If the range quants are smaller than the length of the target returns, several successive digitised samples will be from such returns. Because of the random noise present, the spectrum of samples taken at the centre of the target returns will generally be smaller than that of samples taken from the front or rear part of the target returns; in the latter case, a first trip echo, indeed in more extreme circumstances, can still be classified as a multiple trip echo and be suppressed.